Fuel cell bypass diode structures and attachment methods

ABSTRACT

A fuel cell system includes a fuel cell stack which includes a plurality of fuel cells contacted in series by a plurality of interconnects. The various embodiments provide systems and methods for coupling a fuel cell stack with an electric bypass module within a hot zone. The bypass module may include elements for conducting a current between interconnects in a fuel cell stack and thereby bypass a failed fuel cell that has become a resistive parasitic load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 61/494,937 filed Jun. 9, 2011, which ishereby incorporated herein by reference in its entirety.

BACKGROUND

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. High temperaturefuel cells include solid oxide and molten carbonate fuel cells. Thesefuel cells may operate using hydrogen and/or hydrocarbon fuels. Thereare classes of fuel cells, such as the solid oxide reversible fuelcells, that also allow reversed operation, such that water or otheroxidized fuel can be reduced to unoxidized fuel using electrical energyas an input.

In a high temperature fuel cell system, such as a solid oxide fuel cell(SOFC) system, an oxidizing flow is passed through the cathode side ofthe fuel cell while a fuel flow is passed through the anode side of thefuel cell. The oxidizing flow is typically air, while the fuel flow istypically a hydrogen-rich gas created by reforming a hydrocarbon fuelsource. The fuel cell, operating at a typical temperature between 750°C. and 950° C., enables the transport of negatively charged oxygen ionsfrom the cathode flow stream to the anode flow stream, where the ioncombines with either free hydrogen or hydrogen in a hydrocarbon moleculeto form water vapor and/or with carbon monoxide to form carbon dioxide.The excess electrons from the negatively charged ion are routed back tothe cathode side of the fuel cell through an electrical circuitcompleted between anode and cathode, resulting in an electrical currentflow through the circuit.

Fuel cell stacks may be either internally or externally manifolded forfuel and air. In internally manifolded stacks, the fuel and air isdistributed to each cell using risers contained within the stack. Inother words, the gas flows through openings or holes in the supportinglayer of each fuel cell, such as the electrolyte layer, and gasseparator of each cell. In externally manifolded stacks, the stack isopen on the fuel and air inlet and outlet sides, and the fuel and airare introduced and collected independently of the stack hardware. Forexample, the inlet and outlet fuel and air flow in separate channelsbetween the stack and the manifold housing in which the stack islocated.

Fuel cell stacks are frequently built from a multiplicity of cells inthe form of planar elements, tubes, or other geometries. Fuel and airhas to be provided to the electrochemically active surface, which can belarge. One component of a fuel cell stack is the so called gas flowseparator (referred to as a gas flow separator plate in a planar stack)that separates the individual cells in the stack. The gas flow separatorplate separates fuel, such as hydrogen or a hydrocarbon fuel, flowing tothe fuel electrode (i.e., anode) of one cell in the stack from oxidant,such as air, flowing to the air electrode (i.e., cathode) of an adjacentcell in the stack. Frequently, the gas flow separator plate is also usedas an interconnect which electrically connects the fuel electrode of onecell to the air electrode of the adjacent cell. In this case, the gasflow separator plate which functions as an interconnect is made of orcontains an electrically conductive material.

When a fuel cell fails, it becomes highly resistive. In the case of aSOFC stack, operation of the stack may continue, but the voltage of thestack is increasingly consumed by the voltage drop across the resistiveinterface formed by the failed cell. Bypass diodes have been used infuel cell systems to allow current to bypass the defective fuel cell,but these diodes have been located outside of the fuel cell block or hotzone. In particular, to avoid chemical and thermal degradation of thediodes, the diodes have been located outside of the hot box portion ofthe system that operates at a temperature greater than about 600° C.Jumper wiring has been used to connect the fuel cells to the diodeslocated outside the fuel cell block.

SUMMARY

The various embodiments provide fuel cell systems including a fuel cellstack in a hot zone, the fuel cell stack including a plurality of fuelcells and a plurality of interconnects plates, and a module including aplurality of bypass devices, the module attached to the fuel cell stackand located in the hot zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate example embodiments of theinvention, and together with the general description given above and thedetailed description given below, serve to explain the features of theinvention.

FIGS. 1A and 1B are structural side view diagrams of a fuel cell stackand a bypass module.

FIG. 2A is a structural diagram of a fuel cell stack with protrusions.

FIG. 2B is a structural diagram of a bypass module shaped with recesses.

FIG. 2C is a structural diagram of the bypass module of FIG. 2B attachedto the fuel cell stack of FIG. 2A.

FIG. 2D is a structural diagram of an alternative bypass module attachedto the fuel cell stack of FIG. 2A.

FIG. 3A is a structural diagram of a fuel cell stack with protrusionsaccording to an alternate embodiment.

FIG. 3B is a structural diagram of a bypass module shaped with recessesaccording to an alternate embodiment.

FIG. 3C is a structural diagram of the bypass module of FIG. 3B attachedto the fuel cell stack of FIG. 3A.

FIG. 3D is a structural diagram of an alternative bypass module attachedto the fuel cell stack of FIG. 3A.

FIG. 4A is a structural diagram of a bypass module with protrusionsattached to a fuel cell stack with recesses.

FIGS. 4B, 4C, and 4D are structural diagrams of alternative bypassmodules with openings attached to a fuel cell stack with protrusions.

FIG. 5 is a schematic diagram of a spring connection of a bypass moduleto an interconnect.

FIG. 6 is a structural side view diagram of a fuel cell stack and abypass module with sensors according to an alternative embodiment.

FIG. 7 is a structural side view diagram of a fuel cell stack and abypass module with alternate sensors according to another alternativeembodiment.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.References made to particular examples and implementations are forillustrative purposes, and are not intended to limit the scope of theinvention or the claims.

The various embodiments provide systems and methods for coupling a fuelcell stack with a separate bypass module within a hot zone. The bypassmodule may include bypass elements for conducting a current betweeninterconnects in a fuel cell stack and thereby bypass a failed fuel cellthat has become a resistive parasitic load.

The use of a bypass module enjoys several advantages over other bypassmethods. A bypass module allows for encapsulation of the bypass elementsoutside of the fuel cell stack and without interference with the fuelcell stack assembly process. Bypass modules manufactured outside of thefuel cell stack may be tested externally to ensure proper function inadvance of use or integration. Further, bypass modules containingsemiconductor bypass diodes may be fabricated in the clean roomconditions they may require without placing such a requirement upon thestack itself. Also, any threshold or break-down voltage of the bypasselements in a system could be controlled and changed by swapping outdifferent bypass modules.

The term “fuel cell stack,” as used herein, means a plurality of stackedfuel cells connected in series by a plurality of interconnect plates.For example, FIG. 1A includes a fuel cell stack 100 with fuel cells 102connected in series by interconnects 104. Although the fuel cell stackin FIG. 1 is vertically oriented, fuel cell stacks may be orientedhorizontally or in any other direction.

Fuel cells 102 may include an anode electrode 102A, a solid oxideelectrolyte 102B, and a cathode electrode 102C. The anode electrode maycomprise a cermet comprising a nickel containing phase and a ceramicphase. The nickel containing phase may consist entirely of nickel in areduced state. This phase may form nickel oxide when it is in anoxidized state. Thus, the anode electrode is preferably annealed in areducing atmosphere prior to operation to reduce the nickel oxide tonickel. The nickel containing phase may include other metals inadditional to nickel and/or nickel alloys. The ceramic phase maycomprise a stabilized zirconia, such as yttria and/or scandia stabilizedzirconia and/or a doped ceria, such as gadolinia, yttria and/or samariadoped ceria. The electrolyte may comprise a stabilized zirconia, such asscandia stabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ).Alternatively, the electrolyte may comprise another ionically conductivematerial, such as a doped ceria. The cathode electrode may comprise anelectrically conductive material, such as an electrically conductiveperovskite material, such as lanthanum strontium manganite (LSM). Otherconductive perovskites, such as LSCo, etc., or metals, such as Pt, mayalso be used. The cathode electrode may also contain a ceramic phasesimilar to the anode electrode. The electrodes and the electrolyte mayeach comprise one or more sublayers of one or more of the abovedescribed materials.

Interconnect plates may separate the individual fuel cells in the stack.The interconnect plates may separate fuel, such as a hydrogen and/or ahydrocarbon fuel, flowing to the anode (fuel) electrode of one cell inthe stack, from oxidant, such as air, flowing to the cathode (air)electrode of an adjacent cell in the stack. An interconnect plate maycontain gas flow passages or channels between ribs. The interconnectplate may also electrically connect the anode (fuel) electrode of onecell to the cathode (air) electrode of an adjacent cell, therebyelectrically connecting the cells in series. The interconnect plate maybe made of or may contain electrically conductive material, such as ametal alloy (e.g., chromium-iron alloy) or an electrically conductiveceramic material, which optionally has a similar coefficient of thermalexpansion to that of the solid oxide electrolyte in the cells (e.g., adifference of 0-10%). An electrically conductive contact layer, such asa nickel contact layer, may be provided between the anode electrode andthe interconnect. Another optional electrically conductive contact layermay be provided between the cathode electrode and the interconnect.

The plurality of fuel cells in a fuel cell stack may share a common fuelinlet and exhaust passages or risers. A fuel cell stack may include adistinct electrical entity which contains two end plates on oppositeends of the stack which are connected to power conditioning equipmentand the power (i.e., electricity) output of the stack. Thus, in someconfigurations, the electrical power output from such a distinctelectrical entity may be controlled separately from other stacks. Inother embodiments, multiple stacks may share the same end plates. Inthis case, the stacks may jointly comprise a distinct electrical entity.

A fuel cell stack may be part of a larger fuel cell system forgenerating power. The fuel cell stack may be located in a hot zonewithin such a system. During normal operation, this hot zone may operateat a high temperature, such as a temperature of about 600° C. or more,e.g., 600-1000° C., such as 750-950° C.

Fuel cells typically act as voltage sources in the system. However, fuelcells may have failure modes wherein the fuel cell becomes a resistiveparasitic load. An electric bypass module may be coupled with a fuelcell stack. The fuel cell stack and bypass module may both be located ina hot zone. The bypass module may include elements for conducting acurrent between interconnects in a fuel cell stack and thereby avoid afailed fuel cell that has become a resistive parasitic load.

For example, FIG. 1A illustrates a fuel cell stack 100 with fuel cells102 and interconnects 104. An electric bypass module 120 may includebypass elements 122 electrically connected to contacts 124. Bypasselements 122 are illustrated as diodes, such as semiconductor powerdiodes, but various embodiments may include one or more different typesof bypass elements. FIG. 1B illustrates the bypass module 120 coupledwith the fuel cell stack 100 such that the contacts 124 are inelectrical contact with the interconnects 104. In this way, the bypassmodule 120 may electrically connect two or more interconnects 104 viaone or more bypass elements 122. For example, in FIGS. 1A and 1B eachinterconnect 104 is connected to a second interconnect in electricalcontact with the same fuel cell 102 via contacts 124 and a single bypasselement 122. In this manner, each individual fuel cell may be bypassed.Alternate embodiments may group fuel cells or interconnects in variousways, such as bypassing two or more fuel cells or interconnects with asingle bypass element 122.

In normal operation, fuel cells 102 act as voltage sources. Thereforethe voltage generated by the fuel cell may hold the bypass element 122linking the two interconnects adjacent to the fuel cell in reverse bias.Bypass elements 122 may be selected to effectively be an open circuitunder this reverse bias. If a fuel cell 102 becomes defective, the cell102 may become a highly resistive parasitic load, and a voltage drop maybe formed across the defective cell. The voltage drop may be about 0.5 Vto about 15 V, such as about 1 V to about 5 V. When this voltage dropexceeds a threshold voltage of the bypass element 122 (such as an “on”voltage if the bypass element is a diode), the bypass element 122 may beplaced into forward bias and conduct current between the twointerconnect plates 104 thereby bypassing the defective fuel cell. Thethreshold voltage of the bypass element 122 may vary depending on thebypass module 120 or bypass element 122 used. For example, the thresholdvoltage may be about 0.5 V to about 3 V, such as about 1 V. Preferably,the bypass element 122 is oriented such that the direction of the bypasscurrent (i.e., the current conducted when the bypass element's thresholdvoltage is exceeded) is the same as that of a non-defective cell duringnormal operation. In other words, the bypass direction of the bypasselement 122 is the same direction of current flow as that of anon-defective cell.

The bypass module may be effective to bypass more than one fuel cell. Ifmultiple fuel cells fail, current may pass through multiple bypasselements (or a single bypass element 122 if it connects nonadjacent ormultiple interconnects 104). If two or more failed fuel cells are nextto each other, the current may bypass the fuel cells as well as theinterconnects between the fuel cells.

In various embodiments, the fuel cell stack 100 and the bypass module120 will be coupled within the hot zone and therefore exposed to hightemperatures. The fuel cell stack 100 and the bypass module 120 mayexperience thermal expansion due to the heat. Accordingly, the bypassmodule 120 may be constructed of a material with a similar thermalexpansion rate as the fuel cell stack 100.

In various embodiments, the hot zone may be a hot box 126, as shown inFIG. 1B. A hot box 126 may be a thermally insulated container or housingdesigned to operate with the high operating temperatures of a fuel cellstack 100 (e.g., above 600° C.). A hot box may contain a plurality offuel cell stacks 100 arranged in various ways. For example, a hot boxillustrated in U.S. Pat. No. 7,422,810, which is incorporated herein byreference for a teaching of a hot box and its components, may be used. Abypass module 120 may be coupled with a fuel cell stack 100 within thehot box 126.

The high temperatures of the hot zone also mean that special hightemperature bypass elements may be used. Various embodiments may rely onone or more different types of bypass elements 122. For example, eachbypass element 122 in the figures is illustrated as a diode. However,each bypass element 122 may be any passive or active electrical devicewhich is capable of conducting electricity between at least twointerconnects when at least one fuel cell becomes defective. If a bypasselement 122 is a diode, preferably the diode does not conduct anappreciable amount of current in the reverse bias direction when thecells are not defective. Further, the diode may be capable of operatingat high temperatures within the hot zone, such as about 600-1000° C.Diamond semiconductor diodes are examples of diodes which can operatewith junction temperatures of up to about 1000° C. For example, thediodes disclosed by A. Vescan et al., “Very high temperature operationof diamond Schottky diode,” Electron. Device Letters, IEEE, 18(11):556-558 (1997), which is incorporated herein by reference in itsentirety, may be used. Silicon carbide semiconductor diodes or otherhigh temperature diodes may also be used.

Various alternative embodiments may include break-down dielectricsserving as bypass elements 122. Break-down dielectrics may have poorconductivity until a voltage equal to or greater than a break-downvoltage is applied. As described above, when a fuel cell fails, it maybecome a strong resistive parasitic load instead of a voltage source.This change would cause a higher potential difference across a bypasselement electrically attached to the interconnects on each side of thefuel cell. If the bypass element is a break-down dielectric, the higherpotential difference may be greater than the break-down voltage.Therefore, the break-down dielectric may begin behaving as a conductor(e.g., due to a phase change in the dielectric) and bypass the failedfuel cell. The break-down dielectrics may be selected to break-down at avoltage of about 2 volts. Examples of bypass diode dielectrics includenickel oxide or hafnium oxide layers.

Various embodiments may include antifuses as bypass elements. Anantifuse bypass element may begin with a very high resistance or eveneffectively be an open circuit similar to a diode under reverse bias asdiscussed above. However, the antifuse may create an electricallyconductive path if a threshold voltage or current is exceeded. Similarto a break-down dielectric, the antifuse may bypass a fuel cell whenattached to the interconnects on each side of the fuel cell andsubjected to the higher potential difference caused by the fuel cell'sfailure. Examples of antifuses include silicon oxide or silicon nitridelayers. The high potential causes a conductive link to diffuse throughthe antifuse dielectric 122 from the metal electrodes 128 attached oneach side of the anitfuse dielectric 122 to cause the link to provide aconductive path between the electrodes through the dielectric 122.

Various embodiments may include one or more different ways of couplingthe bypass module with the fuel cell stack. FIG. 2A illustrates a fuelcell stack 200 with fuel cells 102 and interconnects 104 similar to thefuel cell stack 100 of FIGS. 1A and 1B (but rotated 90 degrees around avertical axis). However, fuel cell stack 200 may include protrusions 202on interconnects 104 (e.g., on the edges of interconnect 104 plates). Asshown, protrusions 202 may be in pairs on each interconnect 104 andoffset from the center to align with other protrusions on adjacentinterconnects 104. FIG. 2B illustrates a bypass module 210 having asupport element 211 with recesses 212. Bypass module 210 may includebypass elements 122 with leads or traces 228 to some of the recesses212. FIG. 2C illustrates how bypass module 210 may be attached to thefuel cell stack 200 by coupling the recesses 212 in support element 211on to the protrusions 202. Contact between the protrusions 202 and thecontact portions 224 of the leads, wires, or traces 228 going torecesses 212 may electrically connect bypass elements 122 withinterconnects 104. If the leads or traces 228 of a bypass element 122are arranged to connect two interconnects, as shown in bypass module210, faulty fuel cells between the interconnects may be bypassed.

Various bypass modules 210 may include the support element 211, bypasselements 122 mounted on or supported in support element 211, and aplurality of contacts 224 and leads 228. The support element 211 mayinclude a high temperature ceramic or metal (e.g., a doped ceria orstabilized zirconia, such as yttria or scandia stabilized zirconia, or ametal alloy, such as Cr—Fe alloy, for example Cr-4-6 wt % Fe alloy)which has a similar coefficient of thermal expansion (e.g., 0-10%different) to the solid oxide fuel cells and interconnects. In variousembodiments the support element may be a ceramic (e.g., stabilizedzirconia or doped ceria) circuit board. The support element 211 may be aplate shaped with recesses 212 on its sides such that protrusions 202 ofinterconnects 104 may touch the contacts portions 224 on the surface orinside of support element 211. Preferably, the module is a freestanding,prefabricated module. This means that the support element 211 containingthe bypass elements 122 mounted on or supported in support element 211,and a plurality of contacts 224 and leads 228 are made separately fromthe fuel cell stack, and may be handled or transported as a unitseparately from the fuel cell stack.

FIG. 2D illustrates a fuel cell stack 200 coupled with bypass module 240according to an alternative embodiment. Module 240 includes a “two sidedcomb” shaped support element 211 comprised of a central supportingportion 214 supporting a plurality of “comb teeth” shaped extensions 216on both sides. The adjacent extensions 216 are separated from each otherby recesses 212A, 212B. Module 240 is similar to module 210, butincludes additional recesses 212B that do not couple with protrusions202 from the stack interconnects 104. In other words, recesses 212Acouple with the protrusions 202. However, recesses 212B do not couplewith the protrusions. Thus, every odd or even recess 212A couples with aprotrusion 202 while every other even or odd recess 212B does not couplewith a protrusion 202.

These additional uncoupled recesses 212B may allow the bypass module 240to flex under stress. Specifically, the recesses 212B allow the moduleto flex in the stack 200 stacking direction shown by arrow 280 in FIG.2D to clamp the teeth 216 of the support element 211 onto theprotrusions 202 for support. In further embodiments, the teeth portions216 of support element 211 may temporarily flex perpendicular to thestacking direction (i.e., in and out of the page in FIG. 2D). Inalternate embodiments, the additional recesses 212B of module 240 notused to receive a protrusion 202 may be filled with flexible material,such as a high temperature metal or ceramic foam or felt, or a hightemperature glass sealing material, rather than remain empty.

FIG. 3A illustrates a fuel cell stack 300 with fuel cells 102 andinterconnects 104. The fuel cell stack 300 also includes protrusions202, but unlike fuel cell stack 200, each interconnect 104 may have onlyone protrusion 202. Each interconnect 104 may be manufactured with asingle protrusion 202 offset from the center. When assembling the fuelcell stack 300, some of the interconnects 104 may be rotated one hundredand eighty degrees to form a pattern of protrusions 202 similar to thatin FIG. 3A. Alternatively, two sets of interconnects 104 may befabricated, the first set with a protrusion 202 offset to the right andthe second set with a protrusion 202 offset to the left, and thenassembled into a fuel cell stack by alternating between the two sets.

FIG. 3B illustrates a bypass module 310 similar to bypass module 210except for the arrangement of recesses 212 and bypass elements 122. Thebypass module 310 may be shaped with recesses 212 to align with apattern of protrusions 202 such as that in fuel cell stack 300. Recesses212 may be staggered on the sides of support element 211 (e.g.,alternating left, right, left, etc. as shown in FIG. 3B). The bypasselements 122 in bypass module 310 may be arranged with leads or tracesgoing to recesses 212. FIG. 3C illustrates how bypass module 310 may beattached to the fuel cell stack 300 by coupling the recesses 212 withthe protrusions 202. Similar to the system in FIG. 2C, the contacts 224of leads, wires, or traces 228 may contact the protrusions 202 toelectrically connect the bypass elements 122 with interconnects 104 andbypass faulty fuel cells 102.

FIG. 3D illustrates an alternate embodiment bypass module 320 attachedto a fuel cell stack 300 with protrusions 202. The interconnects 104 inthe stack 300 may have one protrusion 202 per interconnect as shown inFIG. 3A. The bypass module 320 may include teeth 216 that flex to clamparound protrusions 202 similar to the bypass module in FIG. 2D. Thecentral supporting portion 214 of the support element 211 may be shapedor cut out to give the teeth 216 room to flex primarily in the stack 300stacking direction (i.e., up and down in FIG. 3D). The teeth 216 mayflex any suitable amount, such as at least 3 degrees from horizontal,for example 5 to 20 degrees. The modules described herein may have anysuitable dimensions which depend on the stack, interconnect and fuelcell size. For example, the height of the modules (e.g., module 320 inthe up and down direction in FIG. 3D) may be 50 to 500 mm, such as 100to 200 mm, the height of the teeth 216 (e.g., in the up and downdirection in FIG. 3D) may be 0.5 to 10 mm, such as 1 to 3 mm, the widthof the teeth 216 (e.g., in the left and right direction in FIG. 3D) fromtheir edge to the central supporting portion 214 may be 10 to 100 mm,such as 25 to 50 mm, the width (e.g., in the left and right direction inFIG. 3D) of the central supporting element 214 may be 10 to 100 mm, suchas 25 to 50 mm, and the thickness (e.g., in and out of the pagedirection FIG. 3D) may be 0.1 to 10 mm, such as 0.25 to 1 mm.

In other embodiments, support elements 211 may hang on fuel cell stack200 or 300 by inserting protrusions 202 of interconnects 104 intorecesses 212 in support element 211. Alternatively, instead of recesses212, module 200 or 300 or support element 211 may include hooks, clamps,bolts, or other fasteners which are attached directly to interconnects104 or to supports, such as protrusions 202 on the interconnects 104.

FIG. 4A illustrates another alternate embodiment for attaching a fuelcell stack 400 to a bypass module 410. The fuel cell stack 400 mayinclude fuel cells 102 and interconnects 104 with recesses 402. Thebypass module 410 may include protrusions 414. The bypass module 410 mayalso include support element 411 supporting bypass elements 122 withleads or traces 428 running in the protrusions 414. The fuel cell stack400 and bypass module 410 may be attached to each other by coupling theprotrusions 414 and the recesses 402 as shown in FIG. 4A (e.g.,inserting protrusions 414 into recesses 402). The contact portions 424of the traces, wires, or leads 428 may electrically connect the bypasselements 122 and the interconnects 104 so that faulty fuel cells may bebypassed. The contact portions 424 may be located anywhere alongprotrusion 414, such as at the tip.

FIG. 4B illustrates an alternate embodiment in which a bypass module 460includes divots 464 in the support element 411 and the fuel cell stack450 includes interconnects 104 with protrusions 452 that may fit intothe divots 464. The divots (i.e., indentations) 464 may be located inthe flexible teeth of the comb shaped support element 411 to impartadditional flexibility to the module 460 in the stacking direction andperpendicular to the stacking direction.

In another alternative embodiment of the module 470 shown in FIG. 4C,the divots 464 are replaced by through holes 474 which extend all theway through the support element 411. The module 470 and stack 450 shownin FIG. 4C are rotated 90 degrees around a vertical axis compared to themodule and stack in FIG. 4B. Thus, the protrusions 452 extending fromthe interconnects 104 may extend through the bypass module supportelement 411. Preferably, but not necessarily, the bypass module 470support element 411 has the two sided comb shape with recesses 412 andteeth 416 similar to modules 210 and 240 to allow the support element411 to flex in the stack stacking direction and perpendicular to thestacking direction. The recesses 412 may be unfilled (i.e., noprotrusion or material in recesses) or filled with a flexible materialas described with respect to FIG. 2D above. FIG. 4C also illustrates aclamp 480 that may be used to clamp the bypass module 470 to the fuelcell stack 450.

FIG. 4D illustrates an alternate embodiment bypass module 495 attachedto a fuel cell stack 490. Fuel cell stack 490 may have singleprotrusions 452 on alternating sides of each interconnects 104, similarto the arrangement of fuel cell stack 300 shown in FIG. 3A. The bypassmodule 495 may have teeth 416 with through holes 474 similar to thebypass module 470 in FIG. 4C except that the teeth 416 alternate tomatch the single protrusions 452 on each interconnect of the stack 490.The central supporting portion 414 may be cut out similar to that shownin FIG. 2D to allow the support element 411 to flex primarilyperpendicular to the stack stacking direction (i.e., in and out of thepage in FIG. 4D). The element 414 and/or teeth 416 may flexindependently of each other.

Bypass modules may be held in contact with the fuel cell stack by othermechanisms. In various embodiments, the bypass module is held in contactwith the fuel cell stack by one or more spring tension devices, clamps,bolts, etc. As shown in FIG. 5, bypass elements 122 may be held inelectrical contact with protrusions 202 by springs 502 attached at theend of leads, wires, or traces 228. The spring 502 may be a hightemperature metal (e.g., Inconel) coil spring or a high temperaturemetal or ceramic (e.g., stabilized zirconia) leaf spring. Variousembodiments may use one or more weighted elements to press the bypassmodule and fuel cell stack together. Projections and recesses may beomitted with contacts 124 directly contacting exposed edges of theinterconnects 104.

In various embodiments, contacts 124 or leads, wires, or traces 128 ofthe bypass elements 122 of a bypass module, including the alternatebypass modules with recesses or protrusions, may contact the fuel cellstack via contact points 124 coated with various conductive materials.These materials may include platinum, nickel, Inconel, or lanthanumstrontium manganite (LSM).

Various embodiments may involve welding or brazing at the point ofelectrical contact between the bypass module and the fuel cell stack(e.g., contacts 124 may be brazed or welded to interconnects 104).Welding or brazing may improve conduction or help support and maintaincontact between the fuel cell stack and the bypass module.

Further embodiments may include one or more voltage monitoring deviceswithin the bypass module. Voltage monitoring of each of the fuel cellelements may allow for better measurements of system performance and amore precise diagnosis of any system problems. However, the benefit ofmonitoring more voltages is balanced against the rise in cost with eachadditional monitoring device. Previously, a fixed number of monitoringdevices may have been selected for the entire life of a fuel cellsystem. Interchangeable bypass modules may allow adjusting the amount ofmonitoring based on the life cycle of a particular unit. For example,prototype fuel cell stack units may be coupled with bypass modulesequipped with a voltage monitoring device for every fuel cell. Thesemonitoring devices may provide voltage measurements for each cell andmay aid in development. Similarly, a production unit may be coupled witha bypass module with many voltage monitoring devices prior todeployment. In this manner, bypass modules may be tested for defectsprior to being attached to a fuel cell stack. The greater number ofmeasurements allowed by such a bypass module may prevent deployment ofan unreliable unit.

Alternatively, bypass modules with relatively few voltage monitoringdevices may be used for other portions of the unit's life cycle, such asafter deployment in the field (i.e., at the site where the unit islocated to generate electricity). These bypass modules may include anynumber of devices for monitoring the voltage across any number of fuelcells. For example, a bypass module may include a single device tomonitor the voltage across as many as one hundred fuel cells. Alternateembodiment modules may include a voltage monitoring device for each ofmultiple groups of fuel cells. In summary, a fuel cell stack may betested while attached to a first bypass module having a first pluralityof monitoring devices and at least one bypass element. Then the firstbypass module is removed from the fuel cell stack, and a differentsecond bypass module having at least one monitoring device and at leastone bypass element is attached to the fuel cell stack. The fuel cellstack is then operated in the field to generate electricity while thefuel cell stack operation is monitored using the at least one monitoringdevice of the second bypass module. The first bypass module containsmore monitoring devices than the second bypass module. Thus, a firstmodule with more monitoring devices per stack is used during testing ofthe stack and a second module with fewer devices per stack than thefirst module is used during field operation (i.e., electricitygeneration) of the stack.

Bypass modules with different types or number of voltage monitoringdevices may be used at other times in the life cycle as well. Forexample, fuel cell stacks performing in a critical use or othersituations where failures would need to be identified quickly may havebypass modules with many voltage monitoring devices. Total stack lifemay be extended by using modules with many monitoring devices toidentify the first cells to be replaced or repaired in units near theend of the life cycle.

Various embodiments may include different types of voltage monitoringdevices. FIG. 6 illustrates a fuel cell stack 100 attached to a bypassmodule 600 with voltage monitoring. Voltages across the cells 102 andbypass elements 122 may be monitored by running voltage probes 610between each bypass element. These probes 610 may be made of variousdifferent conductive materials that could withstand the temperatures ofthe hot zone such as Ni, Cr, etc. Probes 510 may be mounted on orsupported inside support element 511. The probes 610 may be connectedwith a data logging unit 602 that senses and logs the voltages acrosseach cell or bypass element. Data logging unit may be connected to anoutside device (e.g., a computer) to transfer the logged voltages.Alternatively, data from the data logging unit 602 may be transmittedwirelessly to the outside device to avoid wiring to outside devices.

Although FIG. 6 illustrates voltage probes incorporated into the samemodule 600 as bypass elements 122, these probes may be used separately.Further embodiments include modules with voltage probes 610 but withoutbypass elements 122. Voltage probes 610 may be connected to one or moreinterconnects 104, such as every interconnect, every other interconnect,or various other patterns or random assortments. The number of probescould vary between modules. Similar to bypass elements 122, modules withmore voltage probes 610 may be used during production or development fordiagnostics and modules with fewer probes may be used after deployment.Alternatively, the probe wires may be spot welded to the stack ratherthan being connected using the module.

Further embodiments may include current loops to detect current flowthrough the bypass elements. These current loops 630 may be placedaround one or more bypass elements 122 or groups of bypass elements 122as shown in FIG. 6. When there is bypass current, a voltage may beinduced in the current loop and detected externally. Current loops havethe advantage of detecting fuel cell performance without actual contactwith the fuel cell or fuel cell stack (as is usually done with a voltageprobe) and therefore cannot participate in a short of the fuel cells orstack. The current loops may be attached to data logging or transmittingdevice similar to the data logging unit 602 discussed above or may beconnected to an outside device.

Further embodiments may include high temperature transistors integratedinto the bypass module to form logic gates and create a multiplexer. Amultiplexer may allow monitoring of either voltage probes 610 or currentloops 630 via a multiplexing input signal and allow monitoring of manycells with less signal wires passing into and out of the hot zone.

Various embodiments may include pressure or temperature monitoringdevices integrated into the bypass module. FIG. 7 illustrates a bypassmodule 700 with integrated temperature and/or pressure sensors 704.These devices may connect with ports in the interconnects 104. Thenumber of pressure or temperature monitoring devices per module may varybased on the stack's life cycle as described above for voltagemonitoring devices. The sensors 704 may be connected to a data loggingdevice 702 by probes 710. The data logging device 702 may transmitrecorded pressures or temperatures via wire or wireless connection to anoutside device. The sensors 704 may be mounted on or supported in asupport element 611. Sensors 704 may comprise thermocouples or pressuresensors that contact the fuel cell stack or are located near the fuelcell stack.

The preceding description of the disclosed aspects is provided to enableany person skilled in the art to make or use the present invention.Various modifications to these aspects will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other aspects without departing from the scope of theinvention. Thus, the present invention is not intended to be limited tothe aspects shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A fuel cell system, comprising: a fuel cell stackin a hot zone, the fuel cell stack comprising a plurality of fuel cellsand a plurality of interconnects; and a module comprising a plurality ofbypass elements, the module attached to the fuel cell stack and locatedin the hot zone, wherein at least one of the plurality of bypasselements is electrically connected to at least two of the plurality ofinterconnects so as to bypass at least one of the plurality of fuelcells located between the interconnects, wherein the plurality of bypasselements are mounted on or supported inside a support element, andwherein: the module comprises a freestanding module and the supportelement comprises a support plate which has a substantially similarcoefficient of thermal expansion as the fuel cell stack; the pluralityof interconnects comprise a plurality of protrusions; the supportelement has a comb shape comprising flexible teeth shaped to define aplurality of recesses between the flexible teeth; the bypass module isattached to the fuel cell stack by placing the plurality of protrusionsinto alternating recesses; and recesses without a protrusion areunfilled or filled with a flexible material.
 2. The system of claim 1,wherein the support element is a ceramic circuit board.
 3. The system ofclaim 1, wherein the recesses without the protrusion are unfilled. 4.The system of claim 1, wherein the recesses without the protrusion arefilled with the flexible material.